Update yaml_schema.rst

Fix broken link

docs/source/schemas/yaml_schema.rst

@@ -6,7 +6,7 @@ YAML Schema
 :ref:`YAML Schema <yaml-schema-draft-01>` is a small extension to `JSON Schema
 Draft 4 <http://json-schema.org/draft-04/json-schema-validation.html>`__ that
 adds some features specific to YAML.  `Understanding JSON Schema
-<http://spacetelescope.github.io/understanding-json-schema/>`__ provides a good
+<https://json-schema.org/understanding-json-schema>`__ provides a good
 resource for understanding how to use JSON Schema, and further resources are
 available at `json-schema.org <http://json-schema.org>`__.  A working
 understanding of JSON Schema is assumed for this section, which only describes